1. Field of the Invention
The present invention relates to a power source device that controls output voltage by, for example, switching the input voltage to execute a boosting (step-up) operation.
2. Related Art
Known power source devices include boosting circuits for stepping-up input voltage to a desired output voltage by switching the input voltage, starter circuits for starting boosting circuits at the beginning of the operation of the boosting circuits, and drive circuits for driving the boosting circuits as substitutes for the starter circuits when the output voltage levels of the boosting circuits become equal to or greater than a predetermined level (See Japanese Unexamined Patent Publication No. 2003-92873, for example).
The starter circuits are arranged to generate clock signals with fixed frequencies, and transistors for performing the switching operations of the boosting circuits are on/off controlled using the clock signals (on/off signals).
However, in conventional power source devices, the clock signals generated in the starter circuits are constant. Accordingly, when the boosting circuit is in a starting period of an operation and triggered by starting the starter circuits, wasteful currents flow through the boosting circuits to problematically and wastefully consume electric power.
Further, in recent years switching frequencies have increased and the inductances of coil components have reduced (to smaller sizes) to reduce the size of mounting areas in power source devices used for portable devices such as mobile phones. Therefore, it has become difficult to increase output voltages without increasing the switching frequencies when starting operations from rather low voltages. However, if bipolar transistors are used for starting operations, the switching frequencies are problematically difficult to increase.
Accordingly, an object of the present invention is to provide a power source device capable of reducing power consumption by reducing the wasteful current flowing through the boosting circuit and of a high speed switching operation by a switching element of the boosting circuit even in a low voltage condition when the boosting circuit starts to operate.